This invention is directed to a method and apparatus for molding plastic bottles. More particularly, it is directed to a parison mold and core rod combination which forms a parison having a flat bottom wall with sharply tapered corners, and a shoulder having a substantially straight outer wall and additional plastic at the inner wall thereof which permits a deeper or longer stretch without having the stretch rod damage the parison bottom and/or without deformation in the shoulder of the finished bottles. The shoulder of the finished bottle is slightly thicker than the sidewall.
In recent years substantial effort has been directed to the formation of molecularly oriented plastic bottles as a replacement or partial replacement for glass bottles. According to the prior art, a plastic parison is first injection molded in a parison mold and the parison thereafter stretch/blown into the finished bottle using either of two commonly employed processes, the first being the so-called reheat or two-stage process, and the second being the so-called hot blow or one-stage process. In the reheat or two-stage process the parison, after formation in the parison mold, is removed from the parison mold, cooled to room temperature, and stored for subsequent stretch/blowing in a blow mold into the finished bottle. At the time of the stretch/blowing operation, the parison is reheated and brought to the stretch/blow temperature of the plastic by means of heaters prior to blowing. In the reheat system, if a particular section of the parison is to be at a higher heat, more heat is put into that particular area by using hotter heaters, or the like. In the hot blow or one-stage process after the parison is injection molded in a parison mold it is transferred to a blow mold substantially immediately after formation and while still retaining the heat necessary for the parison to be stretch/blown into the finished bottle. In the hot blow or one-stage process, the cooling of the parison is essentially uniform throughout the parison and, accordingly, provided the thickness of all areas of the parison is the same, all areas of the parison will be at substantially the same temperature.
Various plastics have been suggested for use in the formation of molecularly oriented plastic bottles. The plastics most commonly suggested are polyacrylonitrile or polyethylene terephthalate (PET). Basically the steps of the bottle formation are the same with all plastics. However, each individual plastic will have its own characteristics, and certain modifications--as known to one skilled in the art--are required with respect to handling including modification of temperature conditions, and the like.
As known in the art, the parison to a substantial extent controls the shape and size of the finished bottle. Accordingly, it is essential in order to obtain a bottle without deformations or other imperfections to carefully control the thickness of the parison walls and bottom in addition to controlling the neck finish. Control of the parison during formation has permitted the manufacture of high quality bottles of small and intermediate size with acceptable quality control being possible. However, it has been found in the manufacture of large size bottles, i.e., up to 64 fluid ounces, which are becoming increasingly popular, that problems are encountered including tearing and/or puncturing of the parison bottom by the stretch rod during stretching; deformation at the shoulder of the bottle, and overall poor quality control due primarily to the longer or deeper stretch operation necessary in the formation of bottles of large size.